The present invention relates to a method of treating an aluminum base metal to render it resistant to the corrosion or self anodization mechanism known in the art as water staining. More particularly, it concerns a method of rendering an aluminum base metal resistant to water staining by applying to the surface of the metal a coating which contains an effective amount of a water soluble molybdate salt, preferably sodium molybdate and a water soluble nitrite compound, preferably sodium nitrite.
As used herein the term "aluminum base metal" includes pure aluminum, as well as alloys of aluminum which contain at least fifty weight percent of aluminum. Typical of such alloys are manganese and magnesium alloys of aluminum.
Aluminum and its alloys, i.e., aluminum base metals, are frequently considered to be extremely corrosion resistant. This is attributed to the compact, adherent oxide film which forms on the metal surface. However, aluminum metal can easily experience the water staining form of corrosion or self anodization during shipment and storage. This is due to the ingress of water between coils or sheets of the metal from rain, humidity or temperature fluctuations past the ambient dew point. This water in crevices between aluminum surfaces can provide the conditions for a differential aeration cell. This differential aeration cell consists of regions into which oxygen can readily diffuse, like coil or sheet edges, and regions into which diffusion of oxygen is very limited, like the areas in from the edges of the metal. The oxygen poor region functions as the anode and the oxygen rich region becomes the cathode. Corrosion (i.e., self anodization) can occur in the anodic area, and under these conditions, the water staining form of aluminum corrosion results. Water staining requires nothing more than the presence of water and crevices to form the differential aeration condition. This staining can occur on commercially pure aluminum and all its alloys.
In some case, staining is detrimental by virtue of its unsightliness alone. It can be brownish black, gray or whitish, and can roughen the surface significantly. Staining can also effect mechanical properties of the metal, ease of forming and weldability. Because of these problems, there is great interest in preventing water staining on aluminum.
To date, many types of surface treatments have been tested in attempts to overcome water staining of aluminum. One compound which has been tried for aluminum and other metals is sodium molybdate. However, the use of sodium molybdate by itself on aluminum does not result in satisfactory water staining inhibition.
From the foregoing, it is clear that water staining of aluminum is highly undesirable, and that while numerous techniques have been tried to obviate this problem, no solution has heretofore been advanced which is simple, effective and economical.
Accordingly, the principal object of the present invention is to provide a simple, effective and economical means of rendering aluminum base metal resistant to water staining.
The foregoing and other objects of the instant invention will become apparent to those skilled in the art from a reading of the following specification and claims.